1. Field of the Invention
The invention is directed to a hydraulic propulsion system for a large industrial machine such as an excavator. The machine has at least two hydraulic propulsion motors which are driven from the same source of hydraulic pressure.
2. Description of the Prior Art
Large industrial machines are propelled many times by hydraulic motors. Typically, such machines are provided with internal combustion engines that are used to drive hydraulic pumps. The hydraulic pumps draw hydraulic fluid from a sump and pump the hydraulic fluid into hydraulic lines where it is directed to the propulsion motors and other operating members. Individual three-position directional control valves are used to control the flow of hydraulic fluid to each of the motors, thereby controlling the propulsion motors and other hydraulic motors used for driving the operating members.
In simple hydraulic systems, hydraulic fluid takes the path of least resistance and flows to the area requiring the lowest pressure. This is especially troublesome wherein two hydraulic motors are being used to move a common load, for example two crawler tracks of a crawler excavator, because the low pressure motor will command more hydraulic fluid resulting in an uneven operation of the two motors. To overcome this natural tendency of the hydraulic fluid, compensator valve assemblies are provided to better balance the flow between the two motors by having the high pressure compensator valve assembly meter the low pressure side to even the pressure between the two assemblies.
Although compensator systems work well in most instances, another problem develops when the loads are equal or close to being equal. This situation is noticeable when a crawler operator wants to go in a straight line wherein the tracks need to move equally to accomplish this task. The crawler operator would notice that the crawler would tend to turn to one side or the other as it moves. Therefore, the operator has to continually adjust for this turning movement in the crawler. This situation arises because one of the compensator valve assemblies is dominating the other compensator valve assembly effectively reducing flow through one of the hydraulic motors. This typically happens because the directional control valves are never opened simultaneously and the directional control valve that is opened first creates a dominating compensator valve assembly as it becomes the high pressure compensator valve. The compensator valve assembly associated with the latter opening directional control valve becomes dominated by earlier opening and now high pressure compensator valve assembly and tends to reduce flow to the hydraulic motor to which it is associated. Therefore, the hydraulic motor associated with the first opening directional control valve moves faster than the motor associated with the latter opening directional control valve resulting in a turning movement by the crawler.